Atacama perchlorate as an agricultural contaminant in groundwater: Isotopic and chronologic evidence from Long Island, New York

Perchlorate (ClO₄⁻) is a common groundwater constituent with both synthetic and natural sources. A potentially important source of ClO₄⁻ is past agricultural application of ClO₄⁻-bearing natural NO₃⁻ fertilizer imported from the Atacama Desert, Chile, but evidence for this has been largely circumstantial. Here we report ClO₄⁻ stable isotope data (δ³⁷Cl, δ¹⁸O, and Δ¹⁷O), along with other supporting chemical and isotopic environmental tracer data, to document groundwater ClO₄⁻ contamination sources and history in parts of Long Island, New York. Sampled groundwaters were oxic and ClO₄⁻ apparently was not affected by biodegradation within the aquifers. Synthetic ClO₄⁻ was indicated by the isotopic method in groundwater near a fireworks disposal site at a former missile base. Atacama ClO₄⁻ was indicated in agricultural and urbanizing areas in groundwaters with apparent ages >20 years. In an agricultural area, ClO₄⁻ concentrations and ClO₄⁻/NO₃⁻ ratios increased with groundwater age, possibly because of decreasing application rates of Atacama NO₃⁻ fertilizers and/or decreasing ClO₄⁻ concentrations in Atacama NO₃⁻ fertilizers in recent years. Because ClO₄⁻/NO₃⁻ ratios of Atacama NO₃⁻ fertilizers imported in the past (∼2 × 10⁻³ mol mol⁻¹) were much higher than the ClO₄⁻/NO₃⁻ ratio of recommended drinking-water limits (7 × 10⁻⁵ mol mol⁻¹ in New York), ClO₄⁻ could exceed drinking-water limits even where NO₃⁻ does not, and where Atacama NO₃⁻ was only a minor source of N. Groundwater ClO₄⁻ with distinctive isotopic composition was a sensitive indicator of past Atacama NO₃⁻ fertilizer use on Long Island and may be common in other areas that received NO₃⁻ fertilizers from the late 19th century through the 20th century.